The present invention relates to a process for the preparation of perfluoroalkanols. More particularly, the present invention relates to a process for the preparation of perfluoroalkanols by hydrogenation of the corresponding perfluorinated esters.
It is known from U.S. Pat. Nos. 3,356,747 and 4,072,726 to prepare perfluoroalkanols from perfluorinated esters by vapor phase hydrogenolysis of the esters in the presence of a catalyst based either on a mixed oxide of chromium and copper or on Cu oxide alone. This hydrogenolysis requires substantial heating of the reactants (200.degree. to 350.degree. C.), which results in production costs which are too high to allow consideration of industrial exploitation of the process.
It is also known, from the French patent published under No. 2,060,357, to prepare perfluoroalkanols by hydrogenolysis of perfluorinated esters in water in the presence of a mineral acid such as phosphoric acid. The disadvantage of using an aqueous phase is that in this case partial hydrolysis of the ester occurs, with formation of a perfluorinated acid which in water has a highly corrosive action on the apparatus. Accordingly, this process, for technical reasons, cannot be exploited industrially. It is also known from European Pat. No. 36,939 to prepare perfluoroalkanols by hydrogenolysis of the corresponding perfluorinated esters in the presence of a catalytic composition comprising a metal of group VIII, an alkali metal and an anion radical chosen from among arene radicals and aliphatic alkoxides, at a temperature of about 150.degree. C. The presence of adjuvants such as the alkali metal and the anion radical adds greatly to the cost of production of the trifluoroethanol.
None of the previously described processes allows industrial exploitation, either because the technique is too expensive or because the apparatus can be corroded.
The present invention has succeeded in overcoming these problems and relates to a process for the preparation of perfluoroalkanols characterized in that an ester of a perfluorinated acid corresponding to the formula: ##STR1## in which: n is greater than or equal to 1; and
R is selected from alkyl, alkylphenyl, cycloalkyl, phenyl, halogenoalkyl, halogenoalkylphenyl, halogenocycloalkyl and halogenophenyl groups, optionally mixed with an ester of a nonperfluorinated acid of the formula: ##STR2## in which: n is an integer greater than or equal to 1;
x is an integer between 0 and 2n; PA1 y is an integer between 0 and 2n+1; PA1 z is an integer between 0 and 2n+1; PA1 the sum of x+y+z is equal to 2n+1; and
R has the same meaning as in formula (I), is brought into contact, in the liquid phase, with hydrogen at a total pressure of between about 1 and 300 bars in the presence of a catalyst based on at least one metal selected from nickel, cobalt, copper and the metals of the platinum group.
In formulas (I) and (II), n is preferably equal to or greater than 1 and equal to or less than 12.
In the R substituent in formulas (I) and (II), "alkyl" preferably has from 1 to 6 carbon atoms, more preferably from 1 to 4 carbon atoms, "cycloalkyl" preferably has from 4 to 7 ring carbon atoms, more preferably from 5 to 6 ring carbon atoms, and "halogens" is selected from chloro, fluoro, iodo and bromo and is preferably fluoro.
Representative compounds of formula (I) include ethyltrifluoroacetate, phenyltrifluoroacetate and trifluoroethyl trifluoroacetate. It is preferred to use compounds of formula (I) in which R represents a 1,1-dihydroperfluoroalkane group and more preferably a --CH.sub.2 --C.sub.n F.sub.2n+1 group.
The perfluorinated acid ester preferred according to the invention is trifluoroethyl trifluoroacetate.
The catalyst employed according to the process of the invention may be selected from catalysts based on metals of the platinum group, defined herein as ruthenium, rhodium, iridium, platinum and palladium.
The catalyst of the invention may also include nickel, cobalt and copper. According to a preferred embodiment of the invention, the nickel, cobalt and copper are of the Raney type.
The catalysts used in the present invention may be employed in the metallic state, in the form of an oxide or a salt such as, for example, the chromites or in the form of a mixture of these states. Moreover, they may or may not be deposited on a carrier. Any carrier which is inert under the reaction conditions, such as charcoal, silica or alumina, may be employed. The use of charcoal is very especially preferred.
Among the catalysts, a catalyst based on ruthenium or on rhodium is preferably used. This catalyst is preferably deposited on charcoal.
Preferably, use is made of a quantity of metal, deposited on a carrier, which is less than about 20% by weight, preferably between about 1 and 10% by weight and, more preferably, about 5% by weight, based on the weight of the carrier.
The quantity of catalyst, expressed as the weight of metal employed, is preferably less than about 10% by weight, more preferably from about 1 to 10% by weight, based on the weight of the perfluorinated acid ester. In the case of metals of the platinum group, use is more preferably made of less than about 2% of metal and, more preferably, of less than about 1% of metal, based on the weight of the perfluorinated acid ester. In the case of Raney nickel, use is preferably made of between about 1 and 5% by weight of metal, based on the weight of the perfluorinated acid ester.
The process is preferably carried out at a temperature between the ambient temperature and about 300.degree. C. and more preferably between about 80.degree. and 150.degree. C.
A total pressure of between about 5 and 150 bars is advantageous for carrying out the invention and a pressure of between about 15 and 50 bars is preferred.
The reaction can in particular take place in the presence of solvents which are inert to hydrogenation, such as alkanols, cycloalkanols, alkanes, cycloalkanes, ethers and non-fluorinated esters. As examples of such solvents there may be mentioned trifluoroethanol, cyclohexanol, diisopropyl ether, petroleum cuts corresponding to alkanes of about 6 to 12 carbon atoms and esters of acetic acid such as the acetate of secondary butanol. If the reaction is carried out in the presence of a solvent, it is preferred to use trifluoroethanol, but in the case of the more volatile esters the reaction is preferably carried out without a solvent.
Moreover, when an ester of a perfluorinated acid obtained by, for example, incomplete fluorination of the corresponding perchlorinated acid and containing products corresponding to the general formula (II) is employed, the process makes it possible to avoid the formation of non-perfluorinated alcohols such as, for example, monofluoroethanol, which has an extremely high toxicity even at very low doses. In fact, during the hydrogenation in the presence of the catalysts according to the invention, the esters of non-perfluorinated acids of formula (II) are completely converted to hydrofluoric acid, hydrochloric acid and non-toxic organic compounds.
The acids formed have the effect of slowing down the hydrogenation reaction. Thus, to avoid this adverse effect, a base is advantageously added. If the reaction takes place in the absence of a solvent or in the presence of organic solvents, it is preferred to add an organic base selected from the tertiary amines and the quaternary ammonium hydroxides. The reaction can take place in the presence of an amount of water of which the molar ratio relative to the ester is less than about 10%, and this allows the addition of an inorganic base such as sodium hydroxide, potassium hydroxide, lithium hydroxide and a carbonate. In such a case, it is preferred to add sodium hydroxide or potassium hydroxide.
The invention is particularly indicated for the preparation of trifluoroethanol by hydrogenation of trifluoroethyl trifluoroacetate. Trifluoroethanol is used as a synthetic intermediate in the pharmaceutical and plant protection industries.